!!! Polymers !!!
A. What are Polymers?
Polymers are large molecules whose molecular weight can range from the thousands to millions. Polymers are built up by the repetition of low molecular weight units. ‘poly’ means many and ‘mer’ = parts. For example: Polyethylene is made by ‘joining’ many ethylene molecules together. The process of ‘joining’ is called polymerization.
B. What is Polymer Morphology?
Polymer morphology is the arrangement of the polymer molecules. There are two type of possible arrangements: Amorphous and Crystalline.
C. What are the typical thermal transitions in polymers?
There are three main states in which a polymer can exist and therefore there are two types of typical transitions the polymer can go through. In the Glassy state, the plastic is brittle; in the Rubbery or Viscoelastic state, the plastic is soft and/or tough and in the Melt state the plastic is molten. The transition temperature at which the polymer changes from brittle to soft/tough is called the Glass Transition Temperature (Tg). The transition temperature at which the polymer changes from soft/tough to a melt is called the Melt Temperature or Tm. Amorphous polymers do not show a clear transition into the melt and therefore do not exhibit a Tm. The change is gradual and the polymer softens with increasing temperature. Therefore, in injection molding, amorphous materials (ABS) have a wide melt processing window compared to crystalline materials (Nylons).
!!! Polymer Viscosity !!!
A. What is viscoelasticty?
When liquids are subjected to a force, the deformation is permanent. This property is called viscosity. Viscosity also is an indicator of the resistance to flow. Liquids are said to be viscous. When solids, example: metals, are subjected to a certain limited force, the deformation is not permanent. When the force is released, the solid recovers to its original shape. This property is called elasticity. The word ‘limited’ is used because excessive force causes permanent deformation, another topic of discussion. Plastics exhibit both a viscous component and an elastic component. They are said to be viscoelastic and the behavior is called visoelasticity. This can be visualized with the spring and dash pot model.
B. What are Newtonian and Non-Newtonian fluids
Newtonian Fluid - A fluid whose viscosity is not influenced by the shear rate that is being applied to it. The viscosity remains constant as the shear changes. Shear rate in injection molding is synonymous to injection speed. Non-Newtonian Fluid - A fluid whose viscosity changes with the change in the shear rate that is being applied to it. The viscosity does not remain constant as the shear changes.
C. How is the melt viscosity in polymers different in polymers?
All plastics are non-Newtonian. This means that their viscosity does not remain constant over a given range of shear rates. In the strict sense, the rheological behavior of a plastic is a combination of non – Newtonian and Newtonian behavior. At lower shear rates, the plastic is non-Newtonian but as the shear rate increases, the plastic tends to exhibit a Newtonian behavior. This happens because with increasing shear rate, the polymer molecules start to untangle from each other and start to align themselves in the direction of flow. Refer to the picture below.
!!! Properties of Plastics !!!
Following are some of the important properties of plastics.
a. Density:
Density of a material is the measure of the mass per unit volume and is generally expressed as grams per centimeter cube. Example: Density of General Purpose Polystyrene (GPPS) = 1.06 gms/cc
b. Specific Gravity:
Specific Gravity is the ratio of the density of a material to the density of water. Density of water = 1 gm/cc. Specific gravity of GPPS = (1.06/1) = 1.06 It is a ratio and therefore has no units.
c. Water Absorption:
Water Absorption is the percentage increase in the weight of a material due to absorption of water. Most plastics are hygroscopic (water absorbing). They absorb water and form a chemical bond with it. Example - Nylon, Polyester
This should not be confused with Water Adsorption. Adsorption is attraction of water molecules on the surface of the plastic. The phenomenon is similar to water condensing on a cold soda can. Example: Polyethylene fed into a hopper from a cold silo outside the molding facility
d. Shrinkage:
Shrinkage is the ratio of the volume of the plastic in the melt to the volume of the plastic in its final useable state. When applied to molding, it is expressed as the ratio of the dimension of the mold to the dimension of the part that is molded. Shrinkage is expressed in inches per inches or in percentage
In case of crystalline materials, shrinkage is not the same in the cross flow and the parallel flow direction. Such materials are called anisotropic materials.
In case of amorphous materials, shrinkage is the same in the cross flow and the parallel flow direction. Such materials are called isotropic materials.
e. Melt Flow Rate or Melt Flow Index:
Melt Flow rate (MFR) or Melt Flow Index (MFI) is the amount of material extruded in 10 minutes under certain testing conditions. The plastic is fed into a heated vertical barrel and a known weight is kept on top of the plunger. The amount of plastic that is extruded in 10 minutes is called the MFR or MFI.
MFI and viscosity are inversely related.
High Melt Viscosity = Low MFI
Low Melt Viscosity = High MFI
!!! Injection Moulding !!!
a. What is Injection Molding?
Injection Molding is the technique of injecting molten plastic into a cold mold and forming a part. A schematic of the main elements of a molding machine are shown below. It consists of a hopper that holds the raw plastic pellets. The hopper feeds the barrel with the plastic. The plastic is melted in the barrel and with the help of the screw (piston) is injected into a mold. The mold being colder than the plastic rapidly cools and solidifies the plastic. The mold is then opened and the part is ejected out of the mold.
!!! Books !!!
Polymer materials are made up of “many” (poly) repeating “units”(mers). Polymers are mostly based in carbon, oxygen, and hydrogen. Some have Si, F, Cl, S Polymers are considered a bowl of spaghetti or a bag of worms in constant motion.
Polymer Materials
Polymers are classified broadly as thermoplastic: form after heat. Over and over again. thermoset: set after heat. Only sets once. Can’t reform Thermoplastics crystalline: polymers arranged in a regular order amorphous: polymers arranges randomly like coil Thermosets low molecular weight monomers that crosslink and polymerize to for polymer network Elastomers Can be either thermoplastic and thermoset Thermoset elastomers: natural and synthetic rubbers Thermoplastic elastomers: plastics that mimic rubber (EPDM, TPO, TPE)
Plastic manufacturing processes:
There are many processing methods for polymers. Commercial processing equipment can range from a few thousand dollars to many millions of dollars. In addition to the equipment itself, tooling is generally required to make a particular shape. Most processes involve melting or softening the material and then forcing it into the desired shape. Other processes force a monomer or pre-polymer mixture into the right shape, then polymerize it in-place.
Main processing methods
Extrusion
Process of forming a continuous piece of plastic by forcing it through a shaping orifice with or without the presence of heat. The opening through which the resins are forced gives the product its form, resulting in consistent thickness and gauge control.
Molding
A processing method which forms the rubber in the mold by the application of pressure and heat which shapes and cures the parts.
Thermoforming
Process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.
Recycling
All plastics can be recycled. The extent to which they are recycled depends upon both economic and logistic factors.
Coating
Plastic coating is the process of applying a thermoplastic to the surface of metal items to provide long-term corrosion, impact and chemical resistance whilst offering an attractive decorative finish.
Plastic Extrusion Process:
Plastic Extrusion Process Process of forming a continuous piece of plastic by forcing it through a shaping orifice with or without the presence of heat. The opening through which the resins are forced gives the product its form, resulting in consistent thickness and gauge control
Plastic Extrusion Terms
Extrusion process
Plastic Extrusion Design
Types of plastic extrusion
Sheet Extrusion
Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.
Profile Extrusion
Rubber Profile Extrusion is accomplished by forcing uncured rubber through a die, under heat and pressure, to form a part with a uniform cross section. This uncured rubber is then run through a heating unit to initiate the chemical cross linking reaction that causes the rubber to cure.
Pipe extrusion
Pipe extrusion is defined as a process of forcing the polymer melt through a shaping die (in this case: circular). The extrudate from the die is sized, cooled and the formed pipe is pulled to the winder or a cut off device with the aid of haul off device.
Co-extrusion
The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling.
Blown Film Extrusion
In film blowing a tubular cross-section is extruded through an annular die (usually a spiral die) and is drawn and inflated until the frost line is reached. The extruded tubular profile passes through one or two air rings to cool the material.
Cast Film Extrusion
The cast film process differs from the blown film process through the fast quench and virtual unidirectional orientation capabilities. These characteristics allow a cast film line to operate at higher production rates while producing amazing optics. Applications in food and retail packaging take advantage of these strengths.
Foam Extrusion
During the chemical foam extrusion process plastic resin and chemical foaming agents are mixed and melted. The chemical foaming agent decomposes liberating gas which is dispersed in the polymer melt and expands upon exiting the die. Typically foamed profile extrusions require more intense cooling than solid profiles due to the insulation properties of the foam structure.
Pultrusion
Similar to extrusion but with much higher Strengths- even used to make road bridges. Glass or other fibres are incorporated into the extrusion and so loadings of up to 60% glass can be achieved with very good fibre alignment. Materials are generally thermosetting type materials such as epoxy.
Calendering
Calendering is a process that usually uses four heated rolls rotating at slightly different speeds. Again the material is fed into the rolls, heated and melted, and then shaped into sheet or film. PVC is the most commonly calendered material.
Injection Molding:
Injection molding is the most widely used polymeric fabrication process. It evolved from metal die casting, however, unlike molten metals, polymer melts have a high viscosity and can not simply be poured into a mold. Instead a large force must be used to inject the polymer into the hollow mold cavity. More melt must also be packed into the mold during solidification to avoid shrinkage in the mold. Identical parts are produced through a cyclic process involving the melting of a pellet or powder resin followed by the injection of the polymer melt into the hollow mold cavity under high pressure.
Injection molding can be used to form a wide variety of products. Complexity is virtually unlimited, sizes may range from very small to very large, and excellent control of tolerances is also possible. Most polymers may be injection molded, including thermoplastics, fiber reinforces thermoplastics, thermosetting plastics, and elastomers. Structural injection molding is also possible in which a core and skin may be made of different polymers. Reaction injection molding and liquid injection molding, which differ in the manner of mixing ingredients, involve the injection of liquid polyurethane systems that polymerize within the mold.
Types of Injection Molding
Reaction injection molding
Process for molding polyurethane, epoxy, and other liquid chemical systems. A mixture of two to four components in the proper chemical ratio is accomplished by a high-pressure impingement-type mixing head, from which the mixed material is delivered into the mold at low pressure, where it reacts (cures).
Liquid injection molding
Process that involves an integrated system for proportioning, mixing, and dispensing dual-component liquid resin formulations and directly injecting the resultant mix into a mold which is clamped under pressure. See Injection molding.
Gas assist injection molding
Plastic molding process in which a high-pressure gas is co-injected along with the plastic part to produce discrete internal hollow sections. It differs from blow molding in that one large hollow product is not created; rather, a solid piece with hollow sections is produced.
Co-injection molding
This is a process that creates a skin and core material arrangement in a molded part. The skin material is injected first into the mold cavity, and is immediately followed by a core material. As the skin material flows into the cavity, the material next to the cavity walls freezes and material flows down a center channel. When the core material enters it displaces the skin material in the center of the channel by pushing the skin ahead. As it flows ahead it continues to freeze on the walls producing the skin layer
2-Shot Injection Molding
This technology produces a part with two different raw materials or colors in a single operation. The machine has two injection units: one vertical and one horizontal. By using a rotating mold, the machine automatically produces a substrate of one resin or color and overmolds the part with the second resin or color.
Problems encountered in Injection Molding
Injection Molding Cycle & Process
Injection Molding Troubleshooting
Injection Molding General Processing Guidelines
Blow Molding:
Process of inflating a hot, hollow, thermoplastic preform or parison inside a closed mold so its shape conforms to that of the mold cavity. A wide variety of hollow parts, including plastic bottles, can be produced from many different plastics using this process.
Blow Molding Process
Blow Molding Troubleshooting
Types of Blow molding
Extrusion blow molding
In extrusion blow molding the parison is formed by forcing molten plastic through an annular orifice in a die that is part of the die head assembly. The orifice is formed by the space between the mandrel and the die. Extrusion may be directly from an extruder, or for large parts for which more material is needed than the extruder can continuously provide an accululator is used.
Injection blow molding
Injection blow molding is a two stage process since the parison is produced in a separate operation. In the first process molten plastic is injected into a heated preform mold around a hollow mandrel blow tube or core rod. This is similar to insert injection molding. The workpiece for the second, blow molding, process is the preform-mandrel assembly. The preformed parison is placed in a larger mold cavity for blow molding. Between the preform production and blow blow molding processes a heated preform may be held in a temperature conditioning stage or a cooled preform re-heated. After blow molding the part is stripped from the core rod at an ejection station.
Stretch blow molding
Stretch blow molding produces a part with biaxial molecular alignment. In the process a preform, or parison, elongated mechanically in the mold and than expanded radially in a blowing process. A desirable resulting molecular orientation yields a material with increased strength. This means that products that are strength-based designs can be produced using less material than if they were to be produced using simpler blow molding techniques.